The non-invasive blood pressure measurement function is one of the most basic monitor parameters of the monitors, and a key parameter reflecting the performance characteristic of the monitors. The measurement methods currently utilized in most of the monitors are based on oscillation method. Those methods are popularly applied in clinic because they are convenient in practice with impersonal measurement results and good repeatability.
The oscillation method is a method of inflating the cuff wound around an arm by an air pump so as to interrupt the propagation of pulses in blood vessels, then, deflating the cuff step by step in linear mode (3-5 mmHg/s) or in step mode (6-15 mmHg/step), and converting a pulse signal and a pressure signal, which are transferred to air path via the cuff, into digital signals by the aid of a bridge pressure sensor connected to the air path and corresponding amplifying circuit, filter circuit, A-D converter (analog-to-digital converter), microprocessor control and the like; further, performing proper data process for signals of the pulse wave and the cuff pressure to achieve a series of pulse waves containing a transforming tendency of the pulse waves and a corresponding cuff pressure; and then, using the cuff pressure at the highest pulse as an average pressure of a testee, and calculating the desired results such as a systolic pressure, a diastolic pressure and an average pressure by utilizing some empirical proportional coefficient algorithm.
Because the non-invasive blood pressure measurement method is an indirect measurement method, which requires applying pressure to an upper arm via a cuff for interrupting the propagation of pulses in the blood vessels during measurement process, if the cuff pressure and the measurement time are not properly controlled, the human body might be hurt. For this reason, some detailed international standards have been set for the designing and evaluating of the safety and effectiveness of the blood pressure measurement functions, such as ANSI/AAMI SP10:2002 and IEC 60601-2-30:1999(E) standards. The two standards particularly put forward the requirements for safety as follows:
(1) The maximum cuff pressure obtainable in NORMAL USE shall not exceed 300 mmHg for EQUIPMENT specified for adult PATIENTS (“Adult”) and 150 mmHg for EQUIPMENT specified for use on infant PATIENTS (“Infant”).
(2) In any SINGLE FAULT CONDITION, means shall be provided, functioning independently of the normal pressure control system, which
a) shall prevent the pressure in the cuff from exceeding the maximum NORMAL USE values specified in (1) by more than +10%, and
b) shall activate if the pressure in the cuff exceeds the maximum NORMAL USE values specified in (1) for 15 s.
When being activated, this means shall deflate the cuff to 15 mmHg for adults or 5 mmHg for neonates within 30 s.
SINGLE FAULT CONDITION means any one of the following conditions: a) results in a failure of the normal pressure regulating means, or, b) prevents deflation of the cuff within the specified period, or, c) results in a failure of the normal cuff pressurization timing.
(3) In any mode of operation, including any SINGLE FAULT CONDITION described above, the cuff shall not be inflated above 15 mmHg for more than 180 s for EQUIPMENT specified for use on adult PATIENTS, and shall not be inflated above 5 mmHg for more than 90 s for EQUIPMENT specified for use on infant PATIENTS.
(4) In LONG TERM AUTOMATIC MODE, cuff pressure shall be released for at least 30 s after each period of cuff pressure above 15 mmHg for EQUIPMENT specified for use on adult PATIENTS, or 5 mmHg for EQUIPMENT specified for use on infant PATIENTS, except when the total duration of the alternating inflation/deflation periods does not exceed the maximum inflation time specified in (3). After this the cuff pressure shall be released to below the pressure stated for at least 30 s.
(5) In LONG TERM AUTOMATIC MODE, means shall be provided in any SINGLE FAULT CONDITION as described in (2), functioning independently of the normal timing control system, which, if the deflated period is less than 30 s, will release cuff pressure to below 15 mmHg for EQUIPMENT specified for use on adult PATIENTS, or below 5 mmHg for EQUIPMENT specified for use on infant PATIENTS.
The current safety protection method implemented by the non-invasive blood pressure measurement circuit, such as a Chinese application No. CN03139708.5 entitled “An Electric Non-Invasive Blood Pressure Measurement Apparatus” detects the cuff pressure with an additional pressure sensor, and achieves the overpressure protection under SINGLE FAULT CONDITION by controlling a fast deflation valve with use of a simple comparator. In this method, the “adult” mode and the “infant” mode are distinguished by two comparative levels. Since the calibration of these comparative levels is performed by a potentiometer, which has to be disassembled for calibration, leading to inconvenience. In addition, the overpressure protection point may drift following the voltage of power supply and the drift voltage of the sensor, so the timing function can not be realized independently.
In a Chinese application No. CN200410051173.2 entitled “A Improved Electric Blood Pressure Measurement Method And Apparatus”, a microprocessor is added to realize independent timing. However, the overpressure protection of the apparatus is still based on controlling deflation of the valve with use of a comparator, and the added microprocessor is used only as an independent timer, and can not estimate the internal pressure of the cuff. Although the apparatus can realize simple function of overpressure protection, it cannot estimate threshold value under SINGLE FAULT CONDITION such as 15 mmHg or 5 mmHg for measuring channel fault. In this application, the pressure measurement circuit for overpressure protection has no calibration function, and the drift caused by its sensor and power supply will lead to drift of the overpressure protection point, so larger error may be obtained.